Summer microbial community composition governed by upper-ocean stratification and nutrient availability in northern Marguerite Bay, Antarctica

Abstract

The Western Antarctic Peninsula warmed significantly during the second half of the twentieth century, with a concurrent retreat of the majority of its glaciers, and marked changes in the sea-ice field. These changes may affect summertime upper-ocean stratification, and thereby the seasonal dynamics of phytoplankton and bacteria. In the present study, we examined coastal Antarctic microbial community dynamics by pigment analysis and applying molecular tools, and analysed various environmental parameters to identify the most important environmental drivers. Sampling focussed on the austral summer of 2009-2010 at the Rothera oceanographic and biological Time Series (RaTS) site in northern Marguerite bay, Antarctica. The Antarctic summer was characterized by a salinity decrease (measured at 15 m depth) coinciding with increased meteoric water fraction. Maximum Chl-a values of 35 µg l-1 were observed during midsummer and mainly comprised of diatoms. Microbial community fingerprinting revealed four distinct periods in phytoplankton succession during the summer while bacteria showed a delayed response to the phytoplankton community. Non-metric multidimensional scaling analyses showed that phytoplankton community dynamics were mainly directed by temperature, mixed layer depth and wind speed. Both high and low N/P ratios might have influenced phytoplankton biomass accumulation. The bacterioplankton community composition was mainly governed by Chl-a, suggesting a link to phytoplankton community changes. High-throughput 16S and 18S rRNA amplicon sequencing revealed stable eukaryotic and bacterial communities with regards to observed species, yet varying temporal relative contributions. Eukaryotic sequences were dominated by pennate diatoms in December followed by polar centric diatoms in January and February. Our results imply that the reduction of mixed layer depth during summer, caused by meltwater-related surface stratification, promotes a succession in diatoms rather than in nanophytoflagellates in northern Marguerite Bay, which may favour higher trophic levels.

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

“…In the WAP region, this shift in phytoplankton community seems further fueled by the replacement of sea ice melt by glacial melt as the major source of freshwater (Moline et al, 2004;Mendes et al, 2013). While diatoms are still the major contributor to phytoplankton biomass, cryptophytes, and haptophytes are increasing in the coastal WAP region (Saba et al, 2014;Rozema et al, 2017a). Our current understanding suggests an association of haptophytes with more unstable water columns, while cryptophytes show increased abundances when these water columns are restabilizing due to glacial meltwater injection.…”

“…These blooms are instigated by the melting of sea ice and increase in light availability after the polar winter. With the appearance of deeper mixed layers, the phytoplankton community has reoriented itself toward higher proportions of smaller flagellates such as haptophytes (generally Phaeocystis) and cryptophytes (Arrigo et al, 1999;Montes-Hugo et al, 2009;Kozlowski et al, 2011;Trimborn et al, 2015;Rozema et al, 2017a). In the WAP region, this shift in phytoplankton community seems further fueled by the replacement of sea ice melt by glacial melt as the major source of freshwater (Moline et al, 2004;Mendes et al, 2013).…”

“…Year-round observations of biological and physical processes at this site (15 m depth) started in 1997 Venables et al, 2013;Rozema et al, 2017a). The RaTS site is situated in water ∼520 m deep and ∼2 km from the nearest shore.…”

“…Previous studies have shown a variable, yet generally high, level of phytoplankton biomass dominated by diatoms during summers in this region (Annett et al, 2010;Kozlowski et al, 2011;Rozema et al, 2017a). Phytoplankton biomass, both total and per major taxonomic group, was shown to be related to summer water column stability which, in turn, was driven by winter sea ice cover (Venables et al, 2013;Rozema et al, 2017a). Typically, persistence of summer biomass appears to be regulated by wind mixing events, meltwater input, and nutrient availability (Piquet et al, 2011;Rozema et al, 2017b).…”

“…Here, the year-round Rothera Oceanographic and Biological Time Series (RaTS) program, representing the larger northern Marguerite Bay region, provides the infrastructure and scientific context to understand the dynamics in phytoplankton productivity (Venables and Meredith, 2014). Previous studies have shown a variable, yet generally high, level of phytoplankton biomass dominated by diatoms during summers in this region (Annett et al, 2010;Kozlowski et al, 2011;Rozema et al, 2017a). Phytoplankton biomass, both total and per major taxonomic group, was shown to be related to summer water column stability which, in turn, was driven by winter sea ice cover (Venables et al, 2013;Rozema et al, 2017a).…”